There is an affinity between refrigerants and lubricants such that oil is normally present in the refrigerant circulating in a refrigeration system. In screw compressors, the oil entrained in the refrigerant helps to seal the rotors. In other parts of a refrigeration system, the oil interferes with heat transfer by coating the heat transfer surfaces. In addition to the oil present in the suction gas, oil supplied to lubricate the bearings may leak into the suction gas since oil supplied to the inlet side bearings must be at a pressure greater than suction pressure. Excess oil reduces compressor efficiency. If, for example an initial volume of 1000 cc is to be reduced to 200 cc, a 5:1 compression ratio, the presence of 10 cc of oil will result in 990 cc of refrigerant being reduced to 190 cc, a 5.2:1 compression ratio. Thus a reduced volume of refrigerant will be compressed to an excess pressure. Additionally, an oil separator will be required immediately downstream of the compressor to prevent excess oil circulation or a still will be required to remove and recover excess oil. Where an oil separator is used, a larger unit will be required due to the inlet bearing lubricant flowing into the suction flow.
The present invention has an inlet bearing lubrication system which is isolated from the refrigerant flow through the compressor. During the compression process, the pressure of the trapped volume of refrigerant gas goes from suction to discharge. Accordingly, controlled amounts of gas can be diverted from the trapped volumes over a range of pressures. According to the teachings of the present invention, buffer gas is drawn off from the trapped volumes at a pressure just a few psi higher than the suction pressure. The buffer gas is directed to labyrinth seals at locations between the suction side rotor bearings and suction and between the suction side motor bearings and the motor. The buffer gas flows divide with part going to suction and part to a drain to the oil sump along with the oil flowing from the bearings. The buffer gas flow at the motor side labyrinth seal divides with part going to a drain to the oil sump and part going to the motor chamber which is at suction. Since the buffer gas is just a few psi higher than the compressor suction pressure, there is a minimal impact on the compressor capacity and efficiency.
It is an object of this invention to eliminate, or at least reduce the size of, oil separators or stills in screw compressor refrigeration systems.
It is another object of this invention to provide a separate inlet bearing lubrication system. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, refrigerant gas at a few psi higher than suction is supplied via labyrinth or annular groove type seals located between the suction side bearings and suction and between the suction side motor bearing and the motor. The buffer gas flow divides with part going to suction and part to a drain along the oil flowing from the bearings.